This document outlines five theorems that can prove if a quadrilateral is a parallelogram: if both pairs of opposite sides are congruent; both pairs of opposite angles are congruent; an angle is supplementary to both of its consecutive angles; the diagonals bisect each other; or one pair of opposite sides are congruent and parallel. It also summarizes the key properties of a parallelogram - both pairs of opposite sides are parallel and congruent, both pairs of opposite angles are congruent, or the diagonals bisect each other.